A fluid dynamic bearing device is a bearing device which rotatably supports a shaft member with oil films formed in bearing gaps. The fluid bearing device has characteristics such as high-speed rotation operation, high rotational accuracy, and quietness. In recent years, by taking advantage of those characteristics, the fluid bearing device is suitably used as a bearing device for a motor to be mounted to various electrical apparatuses such as information apparatuses. Specifically, as a bearing device for a motor, the fluid bearing device has been suitably used in the following: a spindle motor for a magnetic disk drive such as an HDD, an optical disk drive for a CD-ROM, CD-R/RW, DVD-ROM/RAM, or the like; a polygon scanner motor of a laser beam printer (LBP); or a fan motor for a PC or the like.
The fluid dynamic bearing device mounted to the spindle motor among the above-mentioned motors includes a shaft member and fixation side members including a bearing sleeve having an inner periphery along which the shaft member is inserted. Examples of the shaft member used in many cases include one provided with a flange portion at one end of a shaft portion thereof. In those cases, a radial bearing gap of a radial bearing portion is formed between an outer peripheral surface of the shaft portion and a surface opposed thereto, and a thrust bearing gap of a thrust bearing portion is formed between an end surface of a flange portion and a surface opposed thereto (refer to Patent Document 1, for example).
As the shaft member provided with a flange, the following are used: one adopting an integrated type in which both the shaft portion and the flange portion are formed integrally with each other by machine work such as cutting; and one adopting a separate type in which the shaft portion and the flange portion separately prepared are integrated with each other by an appropriate means. Examples of the shaft member provided with a flange of the separate type include one in which the one end of the shaft portion is fixed by press-fitting into a through-hole provided at a center of the flange portion as described, for example, in JP 2001-317545 A (Patent Document 2).
Incidentally, during operation of the fluid dynamic bearing device of this type, pressure balance of a lubricating fluid such as a lubricating oil, which fills an inner space, may be disturbed. When the disturbance of the pressure balance occurs between a space defined by one end surface of the flange portion (first thrust bearing gap, for example) and a space defined by another end surface of the flange portion (second thrust bearing gap, for example), there arises a problem of deterioration in rotational accuracy in thrust directions. Problems of this type can be solved by providing a fluid path formed by opening both end surfaces of the flange portion so as to cause a lubricating oil to communicate between the above-mentioned two spaces through the fluid path.